Most packs come in different sizes or have suspensions adjustable to your torso length. The most important thing is to match your torso length with the size of the pack.

Torso:

Measure your torso length by bending your head forward so your chin touches your chest. Feel for the most prominent bump at the back of your neck. Now measure from that point down to an imaginary line connecting the tops of your hips around your back. This works best with a partner's help.